The present invention generally relates to propulsion systems for use onboard spacecraft. The present invention more particularly relates to electric thrusters for positioning and translating such spacecraft in space.
Prior to embarking on a space mission, a spacecraft must be equipped with enough propulsion capability to travel through and maneuver within space in order to carry out the mission. To help provide sufficient propulsion, engineers often include thrusters incorporating electric propulsion systems onboard spacecraft, for electric propulsion systems have been shown to produce exhaust velocities of about 10 to 20 kilometers per second (km/s), or even higher. In producing such high exhaust velocities, the amount of propellant required onboard a spacecraft for a given mission is significantly reduced.
Electric propulsion systems generally fall into three main categories. These categories include electrothermal propulsion systems, electromagnetic propulsion systems, and electrostatic propulsion systems. In electrothermal propulsion systems, a propellant undergoes thermodynamic expansion via controlled thermal heating. In this way, the resultant propellant gas is accelerated until it ultimately reaches a certain exhaust velocity as naturally dictated by gas thermodynamics. In electromagnetic propulsion systems, a propellant is converted into plasma (i.e., an ionized gas), and the plasma is accelerated via an electromagnetic field into a high-velocity exhaust stream. In electrostatic propulsion systems, a propellant is converted into electrically charged ions (i.e., a plasma), and the charged ions are accelerated via an electrostatic field into a high-velocity exhaust stream.
In recent years, the utilization of electrospray techniques as means for ionizing a liquid propellant and producing charged particles for electric propulsion has received considerable attention. In a conventional electrospray technique, a slightly conductive electrolytic liquid is channeled through a capillary needle and emitted from a tip opening in the needle. A strong electrostatic field is applied at the needle tip opening and causes an imbalance of surface force due to the accumulation of charges on the surface of the emitted liquid. If both the flow rate of the liquid and the electric field at the needle tip opening are maintained at proper levels or strengths, a liquid cone commonly referred to as a “Taylor cone” is thereby formed at the needle tip along with a jet issuing from the cone's apex. As the jet travels away from the Taylor cone, the jet eventually becomes unstable and separates into a spray of charged droplets. In this form, the spray of charged droplets, or “electrospray,” is said to be in a “cone-jet mode.”
To date, electrospray techniques have been utilized in thrusters incorporating electrostatic colloid propulsion systems. In general, a colloid thruster is a specific type of electrostatic thruster that utilizes an electrostatic field to accelerate numerous charged liquid drops (i.e., a colloid beam) emitted from a Taylor cone to thereby generate thrust. Typically, an array of emitters consisting of several hundreds of capillary needles is utilized in an individual colloid thruster. When equipped with such emitter arrays, research has shown that colloid thrusters are individually able to deliver thrust levels ranging as high as up to several hundreds of micro-newtons (μN). At such thrust levels, the high-performance propulsion of some small size spacecraft for precision positioning in space is thereby made possible.
In light of the above, it is desirable to further explore the potential benefits of utilizing electrospray techniques in electric propulsion systems for positioning or translating a spacecraft in space.